Page 103 - Wastewater Solids Incineration Systems

72 Wastewater Solids Incineration Systems

Abrasion results in sand particle size being reduced and in the sand exiting the fur-
nace with the flue gases, commonly referred to as elutriation. This sand must be
replaced. Depending on the quality of the feed, the selection of media needs to be
carefully planned. Olivine sand is not desirable in two particular cases. First, if the
feed contains grit, then the bed material can build up and removal of excess bed
material will be required, resulting in unnecessary shutdowns. Second, if the feed
contains high alkali metals, then accumulation of these elements on the olivine bed
can lead to low melting eutectics (Jeffers et. al., 1999).

7.4 Auxiliary Fuel System
Auxiliary fuel is used at startup in a preheat burner or during normal operation by
direct fuel injection into the fluidized sand bed via fuel guns. A wide variety of fuels
including coal, saw dust, and digester gas can be used as supplementary fuel to the
fluid bed incinerator. The only stipulation is that the fuel must be suitable for feeding
in a reliable and controllable manner. Among these fuels, natural gas and no. 2 fuel
oil are most common.

7.4.1 Preheat Burner
When starting up from cold temperatures, it is necessary that a proper burner system
preheats the fluid bed to the temperature at which it is possible to inject fuel to the
bed itself. It is important that proper ignition takes place. In a hot wind box, the pre-
heating is done by a standard industrial oil (or gas) burner, which is placed in the
sidewall of the wind box. The fluidizing air from the heat exchanger (pre-heater) is
heated by mixing with the hot gases from the burner; the resulting hot air then flu-
idizes the sand bed. Heat from the air is then intimately transferred to the fluidizing
sand. In a cold or warm wind box where the preheat burner is installed in the free-
board, the heat transfer from the gas to the bed is less efficient. Therefore, fuel con-
sumption is greater and length of time to startup is higher in a cold wind box than in
a hot wind box.
The air supply to the preheat burner is taken from the outlet side of the fluidizing
air blower. The air is then pressurized further by the combustion air blower to ensure
a burner supply pressure that is approximately 14 kPa (2 psig) greater than the pres-
sure inside the wind box.

7.4.2 Bed Fuel Injection
Once the fluidized bed is sufficiently hot to support instantaneous ignition of the
fuel, the preheat burner can be shut down; preheating is continued by injection of